discourse/app/assets/stylesheets/mobile/upload.scss

.upload-selector {
  input[type="text"]{
    width: 520px;
  }
  input[type="file"] {
    font-size: 14px;
    line-height: 18px;
  }
  .description {
    color: scale-color($primary, $lightness: 50%);
  }
}

.uploaded-image-preview {
  height: 150px;
  background-position: center center;
  background-size: cover;
  background-color: $primary;
}